There are a number of methods of elasticity imaging where elasticity of tissues is evaluated using the data on the strain in a tissue subjected to a given stress. In most of these methods, the information about the strain in the tissue is obtained with the use of conventional ultrasonic imaging techniques.
One approach attempts to determine the shear elasticity of tissue by applying a low frequency vibration (e.g. 100 Hz) to the tissue surface while measuring the amplitude and phase of tissue vibration using ultrasound imaging techniques. See e.g., T. A. Krouskop et al., A Pulsed Doppler Ultrasonic System for Making Non-Invasive Measurement of Mechanical Properties of Soft Tissue, J. Rehab. Res. Dev. Vol. 24, 1-8 (1987); see also R. M. Lerner et al., "Sonoelasticity" Images Derived from Ultrasound Signals in Mechanically Vibrated Tissues, Ultrasound in Med. & Biol. Vol. 16, No. 3, 231-239 (1990), and K. J. Parker et al., U.S. Pat. No. 5,099,848 (1992), and Y. Yamakoshi et al., Ultrasonic Imaging of Internal Vibration of Soft Tissue Under Forced Vibration, IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, Vol. 7, No. 2, 45-53 (1990).
A method for measuring and imaging tissue elasticity is described in Ophir et al., U.S. Pat. No. 5,107,837 (1992), and Ophir et al., U.S. Pat. No. 5,293,870 (1994). This method includes emitting ultrasonic waves along a path into the tissue and detecting an echo sequence resulting from the ultrasonic wave pulse. The tissue is then compressed (or alternatively decompressed from a compressed state) along the path and during such compression, a second pulse of ultrasonic waves is sent along the path into the tissue. The second echo sequence resulting from the second ultrasonic wave pulse is detected and then the differential displacement of selected echo segments of the first and second echo sequences are measured. A selected echo segment of the echo sequence, i.e., reflected RF signal, corresponds to a particular echo source within the tissue along the beam axis of the transducer. Time shifts in the echo segment are examined to measure compressibilities of the tissue regions.
More recently, M. O'Donnell et al., Internal displacement and strain imaging using ultrasonic speckle tracking. IEEE Transactions on Ultrasonic Ferroelectrics and Frequency Control; Vol. 41, 314-325, (1994), have used Fourier-based speckle tracking techniques to improve the strain measurements in the tissue produced by an external mechanical load. It has been shown that by applying incremental deformations and collecting a large set of complex B-Scan images, the strain signal-to-noise ratio can be significantly improved.
The various approaches differ both in how the medium is stressed and in how the measured ultrasound signals are processed. T. Sugimoto et al., Tissue Hardness Measurement Using The Radiation Force of Focused Ultrasound. IEEE 1990 Ultrasonic Symposium, 1377-80 (1990), suggested a measurement technique of hardness, where the radiation force of focused ultrasound was used to generate the deformation of the tissue, and deformation was measured as a function of time by a conventional pulse-echo technique. The radiation pressure of focused ultrasound exerts such a substantial mechanical stress in the media that even at the exposure levels typical for medical ultrasonic pulse-echo imaging devices significant acoustical streaming can be induced in a liquid. See H. C. Starritt et al., An experimental investigation of streaming in pulsed diagnostic ultrasound beams. Ultrasound Med Biol; Vol. 15, 363-373 (1989).
The use of radiation pressure of focused ultrasound to exert force in medium to obtain information on its mechanical properties has been described. See S. Dymling et al., Acoustic Method For Measuring Properties of a Mobile Medium. U.S. Pat. No. 5,056,357 (1991). They derived the information on the viscosity of the fluids by measuring the velocity of the streaming induced by radiation pressure using Doppler ultrasound.